1. Field of the Invention
The present relates to an alternating current (AC) circuit, especially, relates to an AC circuit in which a capacitor is used for electrically connecting circuit elements.
2. Related Prior Art
An AC circuit operating in a high frequency band uses specific circuit elements such as transmission line types of a micro-strip and a co-planar, and a passive element with a chip shape. In order to electrically connect active elements, such as an amplifier, the AC coupling circuit must be set between them because direct current (DC) levels of these active elements are different to each other. For example, in the case that two amplifier are serially connected, the output DC level of the precedent amplifier often mismatches to the input DC level of the succeeding one. The AC coupling circuit generally includes a coupling capacitor set on the transmission line connecting the preceding circuit to the succeeding one.
FIG. 1 shows a conventional arrangement of the coupling capacitor. In FIG. 1, an integrated circuit 11 with a bare chip shape is connected, by a plural bonding wires 24a to 24c, to a wiring substrate 11 on which two ground patterns 22a and 22b sandwich a transmission line 21 therebetween. The transmission line 21 is split and the capacitor 23 is mounted on the line 21 so as to connect the split line.
In such AC coupling circuit shown in FIG. 1, even in the case that the transmission line 21 and the ground patterns adjacent thereto is so designed to adjust the transmission impedance, an impedance mismatching may occur at points the coupling capacitor is connected to the transmission line. This causes a reflection, loss and deterioration of the signal transmitted thereon.
Further, a gap between the substrate 11 and the IC 12 must be secured at least 200 μm, which causes bonding wires 24a to 24c longer than 400 μm or amount to 500 μm. Such long and slim bonding wire behaves as an inductor. For example, a ribbon wire with a 50 μm in width and a 20 μm in thickness has a parasitic inductance of about 200 pH. The parasitic inductance inherently accompanied with the bonding wire degrades quality of high frequency signals by the resonance with input capacitance of the IC 12.